Galaxy groups and clusters

Galaxy groups and clusters are the largest known gravitationally bound objects to have arisen thus far in the process of cosmic structure formation.[2] They form the densest part of the large-scale structure of the Universe. In models for the gravitational formation of structure with cold dark matter, the smallest structures collapse first and eventually build the largest structures, clusters of galaxies. Clusters are then formed relatively recently between 10 billion years ago and now. Groups and clusters may contain ten to thousands of individual galaxies. The clusters themselves are often associated with larger, non-gravitationally bound, groups called superclusters.

A scattering of spiral and elliptical galaxies
MACS J0152.5-2852 is a massive galaxy cluster. Almost every object seen in the image is a galaxy, each containing billions of stars.[1]

Groups of galaxies

Map of the positions of thousands of galaxies in the VIPERS survey
Map of the positions of thousands of galaxies in the VIPERS survey[3]

Groups of galaxies are the smallest aggregates of galaxies. They typically contain no more than 50 galaxies in a diameter of 1 to 2 megaparsecs (Mpc)(see 1022 m for distance comparisons). Their mass is approximately 1013 solar masses. The spread of velocities for the individual galaxies is about 150 km/s. However, this definition should be used as a guide only, as larger and more massive galaxy systems are sometimes classified as galaxy groups.[4] Groups are the most common structures of galaxies in the universe, comprising at least 50% of the galaxies in the local universe. Groups have a mass range between those of the very large elliptical galaxies and clusters of galaxies.[5]

Our own Galaxy, the Milky Way, is contained in the Local Group of more than 40 galaxies.[6]

In July 2017 S. Paul, R. S. John et al. define clear distinguishing parameters for classifying ‘galaxy groups’ and ‘clusters’ on the basis of scaling laws that they followed [7]. According to this paper, those large scale structures in the universe with mass less than 8 × 1013 solar mass is classified as Galaxy group.

Clusters of galaxies

Rich scattering of galaxies was captured by the MPG/ESO telescope.

Clusters are larger than groups, although there is no sharp dividing line between the two. When observed visually, clusters appear to be collections of galaxies held together by mutual gravitational attraction. However, their velocities are too large for them to remain gravitationally bound by their mutual attractions, implying the presence of either an additional invisible mass component, or an additional attractive force besides gravity. X-ray studies have revealed the presence of large amounts of intergalactic gas known as the intracluster medium. This gas is very hot, between 107K and 108K, and hence emits X-rays in the form of bremsstrahlung and atomic line emission.

ACO 3341
Galaxy cluster ACO 3341 seen by VLT's VIMOS

The total mass of the gas is greater than that of the galaxies by roughly a factor of two. However, this is still not enough mass to keep the galaxies in the cluster. Since this gas is in approximate hydrostatic equilibrium with the overall cluster gravitational field, the total mass distribution can be determined. It turns out the total mass deduced from this measurement is approximately six times larger than the mass of the galaxies or the hot gas. The missing component is known as dark matter and its nature is unknown. In a typical cluster perhaps only 5% of the total mass is in the form of galaxies, maybe 10% in the form of hot X-ray emitting gas and the remainder is dark matter. Brownstein and Moffat[8] use a theory of modified gravity to explain X-ray cluster masses without dark matter. Observations of the Bullet Cluster are the strongest evidence for the existence of dark matter;[9][10][11] however, Brownstein and Moffat[12] have shown that their modified gravity theory can also account for the properties of the cluster.

Observational methods

Galaxy Cluster LCDCS-0829
Galaxy Cluster LCDCS-0829 acting like a giant magnifying glass. This strange effect is called gravitational lensing.

Clusters of galaxies have been found in surveys by a number of observational techniques and have been studied in detail using many methods:

  • Optical or infrared: The individual galaxies of clusters can be studied through optical or infrared imaging and spectroscopy. Galaxy clusters are found by optical or infrared telescopes by searching for overdensities, and then confirmed by finding several galaxies at a similar redshift. Infrared searches are more useful for finding more distant (higher redshift) clusters.
  • X-ray: The hot plasma emits X-rays that can be detected by X-ray telescopes. The cluster gas can be studied using both X-ray imaging and X-ray spectroscopy. Clusters are quite prominent in X-ray surveys and along with AGN are the brightest X-ray emitting extragalactic objects.
  • Radio: A number of diffuse structures emitting at radio frequencies have been found in clusters. Groups of radio sources (that may include diffuse structures or AGN) have been used as tracers of cluster location. At high redshift imaging around individual radio sources (in this case AGN) has been used to detect proto-clusters (clusters in the process of forming).
  • Sunyaev-Zel'dovich effect: The hot electrons in the intracluster medium scatter radiation from the cosmic microwave background through inverse Compton scattering. This produces a "shadow" in the observed cosmic microwave background at some radio frequencies.
  • Gravitational lensing: Clusters of galaxies contain enough matter to distort the observed orientations of galaxies behind them. The observed distortions can be used to model the distribution of dark matter in the cluster.

Temperature and density

Most remote mature cluster
The Most Distant Mature Galaxy Cluster[13] taken with ESO's Very Large Telescope in Chile and with NAOJ's Subaru Telescope in Hawaii

Clusters of galaxies are the most recent and most massive objects to have arisen in the hierarchical structure formation of the Universe and the study of clusters tells one about the way galaxies form and evolve. Clusters have two important properties: their masses are large enough to retain any energetic gas ejected from member galaxies and the thermal energy of the gas within the cluster is observable within the X-Ray bandpass. The observed state of gas within a cluster is determined by a combination of shock heating during accretion, radiative cooling, and thermal feedback triggered by that cooling. The density, temperature, and substructure of the intracluster X-Ray gas therefore represents the entire thermal history of cluster formation. To better understand this thermal history one needs to study the entropy of the gas because entropy is the quantity most directly changed by increasing or decreasing the thermal energy of intracluster gas.[14]

List of groups and clusters

Name / Designation Notes
Local Group The group where the Milky Way, including the Earth is located
Virgo Cluster This cluster of galaxies is the nearest one to us

See also


  1. ^ "A scattering of spiral and elliptical galaxies". ESA/Hubble Picture of the Week. Retrieved 25 September 2013.
  2. ^ Voit, G.M.; "Tracing cosmic evolution with clusters of galaxies"; Reviews of Modern Physics, vol. 77, Issue 1, pp. 207-258
  3. ^ "Huge Map of the Distant Universe Reaches Halfway Point". ESO. Retrieved 2 April 2013.
  4. ^ UTK Physics Dept. "Groups of Galaxies". University of Tennessee, Knoville. Retrieved September 27, 2012.
  5. ^ Muñoz, R. P.; et al. (11 December 2012). "Dynamical analysis of strong-lensing galaxy groups at intermediate redshift". Astronomy & Astrophysics (published April 2013). 552: 18. arXiv:1212.2624. Bibcode:2013A&A...552A..80M. doi:10.1051/0004-6361/201118513. A80.
  6. ^ Mike Irwin. "The Local Group". Retrieved 2009-11-07.
  7. ^ S. Paul, R. S. John, P. Gupta, H. Kumar (2017). "Understanding 'galaxy groups' as a unique structure in the universe". Monthly Notices of the Royal Astronomical Society. 471 (1): 2–11. arXiv:1706.01916. Bibcode:2017MNRAS.471....2P. doi:10.1093/mnras/stx1488.CS1 maint: Multiple names: authors list (link)
  8. ^ Brownstein, J. R.; Moffat, J. W. (2006). "Galaxy Cluster Masses Without Non-Baryonic Dark Matter". Monthly Notices of the Royal Astronomical Society. 367 (2): 527–540. arXiv:astro-ph/0507222. Bibcode:2006MNRAS.367..527B. doi:10.1111/j.1365-2966.2006.09996.x.
  9. ^ Markevitch; Gonzalez; Clowe; Vikhlinin; David; Forman; Jones; Murray; Tucker (2004). "Direct constraints on the dark matter self-interaction cross-section from the merging galaxy cluster 1E0657-56". Astrophys. J. 606 (2): 819–824. arXiv:astro-ph/0309303. Bibcode:2004ApJ...606..819M. doi:10.1086/383178.
  10. ^ Coe, Dan; Benítez, Narciso; Broadhurst, Tom; Moustakas, Leonidas A.; Benítez; Broadhurst; Moustakas (2010). "A High-resolution Mass Map of Galaxy Cluster Substructure: LensPerfect Analysis of A1689". The Astrophysical Journal. 723 (2): 1678–1702. arXiv:1005.0398. Bibcode:2010ApJ...723.1678C. doi:10.1088/0004-637X/723/2/1678.CS1 maint: Multiple names: authors list (link)
  11. ^ McDermott, Samuel D.; Yu, Hai-Bo; Zurek, Kathryn M.; Yu; Zurek (2011). "Turning off the lights: How dark is dark matter?". Physical Review D. 83 (6): 063509. arXiv:1011.2907. Bibcode:2011PhRvD..83f3509M. doi:10.1103/PhysRevD.83.063509.CS1 maint: Multiple names: authors list (link)
  12. ^ Brownstein, J. R.; Moffat, J. W. (2007). "The Bullet Cluster 1E0657-558 evidence shows Modified Gravity in the absence of Dark Matter". Monthly Notices of the Royal Astronomical Society. 382 (1): 29–47. arXiv:astro-ph/0702146v3. Bibcode:2007MNRAS.382...29B. doi:10.1111/j.1365-2966.2007.12275.x.
  13. ^ "The Most Distant Mature Galaxy Cluster". ESO Science Release. ESO. Retrieved 9 March 2011.
  14. ^ Galaxies. Wikimedia Foundation. p. 55.

Further reading

Coma Cluster

The Coma Cluster (Abell 1656) is a large cluster of galaxies that contains over 1,000 identified galaxies. Along with the Leo Cluster (Abell 1367), it is one of the two major clusters comprising the Coma Supercluster. It is located in and takes its name from the constellation Coma Berenices.

The cluster's mean distance from Earth is 99 Mpc (321 million light years). Its ten brightest spiral galaxies have apparent magnitudes of 12–14 that are observable with amateur telescopes larger than 20 cm. The central region is dominated by two supergiant elliptical galaxies: NGC 4874 and NGC 4889. The cluster is within a few degrees of the north galactic pole on the sky. Most of the galaxies that inhabit the central portion of the Coma Cluster are ellipticals. Both dwarf and giant ellipticals are found in abundance in the Coma Cluster.

Coma Filament

Coma Filament is a galaxy filament. The filament contains the Coma Supercluster of galaxies and forms a part of the CfA2 Great Wall.

Dwarf elliptical galaxy

Dwarf elliptical galaxies, or dEs, are elliptical galaxies that are smaller than ordinary elliptical galaxies. They are quite common in galaxy groups and clusters, and are usually companions to other galaxies.

Galaxy cluster

A galaxy cluster, or cluster of galaxies, is a structure that consists of anywhere from hundreds to thousands of galaxies that are bound together by gravity with typical masses ranging from 1014–1015 solar masses. They are the largest known gravitationally bound structures in the universe and were believed to be the largest known structures in the universe until the 1980s, when superclusters were discovered. One of the key features of clusters is the intracluster medium (ICM). The ICM consists of heated gas between the galaxies and has a peak temperature between 2–15 keV that is dependent on the total mass of the cluster. Galaxy clusters should not be confused with star clusters, such as open clusters, which are structures of stars within galaxies, or with globular clusters, which typically orbit galaxies. Small aggregates of galaxies are referred to as galaxy groups rather than clusters of galaxies. The galaxy groups and clusters can themselves cluster together to form superclusters.

Notable galaxy clusters in the relatively nearby Universe include the Virgo Cluster, Fornax Cluster, Hercules Cluster, and the Coma Cluster. A very large aggregation of galaxies known as the Great Attractor, dominated by the Norma Cluster, is massive enough to affect the local expansion of the Universe. Notable galaxy clusters in the distant, high-redshift Universe include SPT-CL J0546-5345 and SPT-CL J2106-5844, the most massive galaxy clusters found in the early Universe. In the last few decades, they are also found to be relevant sites of particle acceleration, a feature that has been discovered by observing non-thermal diffuse radio emissions, such as radio halos and radio relics. Using the Chandra X-ray Observatory, structures such as cold fronts and shock waves have also been found in many galaxy clusters.


A Group is a number of people or things that are located, gathered, or classed together.

Heidelberg University Faculty of Physics and Astronomy

The Faculty of Physics and Astronomy is one of twelve faculties at the University of Heidelberg. It comprises the Kirchhoff Institute of Physics, the Institute of Physics, Theoretical Physics, Environmental Physics and Theoretical Astrophysics.

List of galaxy groups and clusters

This page lists some galaxy groups and galaxy clusters.

Defining the limits of galaxy clusters is imprecise as many clusters are still forming. In particular, clusters close to the Milky Way tend to be classified as galaxy clusters even when they are much smaller than more distant clusters.

Location of Earth

Knowledge of the location of Earth has been shaped by 400 years of telescopic observations, and has expanded radically in the last century. Initially, Earth was believed to be the center of the Universe,

which consisted only of those planets visible with the naked eye and an outlying sphere of fixed stars. After the acceptance of the heliocentric model in the 17th century, observations by William Herschel and others showed that the Sun lay within a vast, disc-shaped galaxy of stars. By the 20th century, observations of spiral nebulae revealed that our galaxy was one of billions in an expanding universe, grouped into clusters and superclusters. By the end of the 20th century, the overall structure of the visible universe was becoming clearer, with superclusters forming into a vast web of filaments and voids. Superclusters, filaments and voids are the largest coherent structures in the Universe that we can observe. At still larger scales (over 1000 megaparsecs) the Universe becomes homogeneous meaning that all its parts have on average the same density, composition and structure.Since there is believed to be no "center" or "edge" of the Universe, there is no particular reference point with which to plot the overall location of the Earth in the universe. Because the observable universe is defined as that region of the Universe visible to terrestrial observers, Earth is, because of the constancy of the speed of light, the center of Earth's observable universe. Reference can be made to the Earth's position with respect to specific structures, which exist at various scales. It is still undetermined whether the Universe is infinite. There have been numerous hypotheses that our universe may be only one such example within a higher multiverse; however, no direct evidence of any sort of multiverse has ever been observed, and some have argued that the hypothesis is not falsifiable.

Lynx–Ursa Major Filament

Lynx–Ursa Major Filament (LUM Filament) is a galaxy filament.The filament is connected to and separate from the Lynx–Ursa Major Supercluster.

Marie Machacek

Marie E. Machacek is an astrophysicist conducting research in the High Energy Astrophysics Division of the Smithsonian Astrophysical Observatory. Her current research explores interacting galaxies and the evolution of galaxies in galaxy groups and clusters. She is also the current coordinator for the SAO Astronomy Intern Program.

Machacek was co-author of a Harvard–Smithsonian Center for Astrophysics (CFA) study concerning galaxy NGC 5195 presented in January 2016 at the 227th meeting of the American Astronomical Society (AAS).

NGC 5846

NGC 5846 is an elliptical galaxy located in the constellation Virgo. It is located at a distance of circa 90 million light years from Earth, which, given its apparent dimensions, means that NGC 5846 is about 110,000 light years across. It was discovered by William Herschel on February 24, 1786. It lies near 110 Virginis and is part of the Herschel 400 Catalogue.

NGC 7012

NGC 7012 is a large, bright elliptical galaxy located about 380 million Light-years away from Earth in the constellation Microscopium NGC 7012 was discovered by astronomer John Herschel on July 1, 1834.

Outline of Earth

The following outline is provided as an overview of and topical guide to the planet Earth:

Earth – third planet from the Sun, the densest planet in the Solar System, the largest of the Solar System's four terrestrial planets, and the only astronomical object known to harbor life.

Perseus–Pegasus Filament

Perseus–Pegasus Filament is a galaxy filament containing the Perseus-Pisces Supercluster and stretching for roughly a billion light years (or over 300/h Mpc). Currently, it is considered to be one of the largest known structures in the universe. This filament is adjacent to the Pisces–Cetus Supercluster Complex.

Radio galaxy

Radio galaxies and their relatives, radio-loud quasars and blazars, are types of active galaxy nuclei that are very luminous at radio wavelengths, with luminosities up to 1039 W between 10 MHz and 100 GHz. The radio emission is due to the synchrotron process. The observed structure in radio emission is determined by the interaction between twin jets and the external medium, modified by the effects of relativistic beaming. The host galaxies are almost exclusively large elliptical galaxies. Radio-loud active galaxies can be detected at large distances, making them valuable tools for observational cosmology. Recently, much work has been done on the effects of these objects on the intergalactic medium, particularly in galaxy groups and clusters.

Somak Raychaudhury

Somak Raychaudhury (Bengali: সোমক রায়চৌধুরী) is an Indian astrophysicist. He is the Director of the Inter-University Centre for Astronomy and Astrophysics (IUCAA), Pune. He is on leave from Presidency University, Kolkata, India, where he is a Professor of Physics, and is also affiliated to the University of Birmingham, United Kingdom. He is known for his work on stellar mass black holes and supermassive black holes. His significant contributions include those in the fields of gravitational lensing, galaxy dynamics and large-scale motions in the Universe, including the Great Attractor.

Stellar drift

Stellar drift, or the motion of stars, is a necessary result of the lack of an absolute reference frame in special relativity.

Nothing in space stands still—more precisely, standing still is meaningless without defining what "still" means. Most galaxies have been moving away ever since the Big Bang, as explained by the metric expansion of space. Galaxy motion is also influenced by galaxy groups and clusters. Stars orbit moving galaxies, and they also orbit moving star clusters and companion stars. Planets orbit their moving stars.

Stellar drift is measured by two components: proper motion (multiplied by distance) and radial velocity. Proper motion is a star's motion across the sky, slowly changing the shapes of constellations over thousands of years. It can be measured using a telescope to detect small movements over long periods of time. Radial velocity is how fast a star approaches or recedes from us. It is measured using redshift. Both components are complicated by the Earth's orbit around the Sun, so the motions of stars are described relative to the Sun, not the Earth (kinematics of stars).

Ursa Major Filament

Ursa Major Filament is a galaxy filament. The filament is connected to the CfA Homunculus, a portion of the filament forms a portion of the "leg" of the Homunculus.

Virgo Supercluster

The Virgo Supercluster (Virgo SC) or the Local Supercluster (LSC or LS) is a mass concentration of galaxies containing the Virgo Cluster and Local Group, which in turn contains the Milky Way and Andromeda galaxies. At least 100 galaxy groups and clusters are located within its diameter of 33 megaparsecs (110 million light-years). The Virgo SC is one of about 10 million superclusters in the observable universe and is in the Pisces–Cetus Supercluster Complex, a galaxy filament.

A 2014 study indicates that the Virgo Supercluster is only a lobe of an even greater supercluster, Laniakea, a larger, competing referent of Local Supercluster centered on the Great Attractor.

Active nuclei
Energetic galaxies
Low activity
See also

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